1. Field of the Invention
The present invention relates to a solar cell adapted for use as a power source for various electronic equipment or as a power source for power generating facilities, and more particularly a large-grain polycrystalline solar cell with a high energy conversion efficiency and with satisfactory mass producibility, and a method for producing such solar cell.
2. Related Background Art
Solar cells have been utilized as power sources for various equipment. Such solar cells include a p-n junction as the functional part, and the semiconductor constituting such p-n junction is generally silicon. Monocrystalline silicon is preferable in terms of efficiency for converting the light energy into electric power, but amorphous silicon is considered more advantageous in formation of large-area devices and in cost reduction. In recent years, there has been investigated the use of polycrystalline silicon, for the purpose of attaining a low cost, as with amorphous silicon, and a high energy conversion efficiency as with monocrystalline silicon. However, in the conventionally proposed method, utilizing plate-shaped materials obtained by slicing a block of polycrystal, it has been difficult to reduce the thickness of the device below 0.3 mm. Consequently, the device becomes excessively thicker than required for sufficient light absorption, and the material has not effectively been utilized. Thus, for cost reduction, formation of a sufficiently thin structure is essential.
For this purpose, there has been investigated the formation of a thin film of polycrystalline silicon by thin film forming technologies such as chemical vapor deposition (CVD), but such methods have only provided crystals with a grain size less then 1/10 micron, so that the energy conversion efficiency is even lower than that obtained with the plate sliced from a polycrystalline silicon block.
Also, there has been reported a so-called "abnormal grain growing method" (Yasuo Wada and Shigeru Nishimatsu, Journal of Electrochemical Society, Solid-State Science and Technology, 125 (1978) 1499) for expanding the grain size even in excess of 10 times the film thickness, by introducing atoms of an impurity such as phosphorus by ion implantation to a supersaturated state into the above-mentioned polycrystalline thin silicon film, formed by CVD, and then annealing said film at a high temperature. However, such film cannot be used as the active layer for generating the photocurrent because of the excessively high impurity concentration. Also, there has been an attempt to irradiate a polycrystalline thin silicon film with a laser beam, thereby enlarging the crystal grain size by fusion and recrystallization, but such method cannot achieve sufficient cost reduction and stable manufacture.
The above described situation exists not only with silicon but also with compound semiconductors.
On the other hand, a solar cell as shown in FIG. 3 can be obtained by forming, on a substrate surface, a different material having a nucleation density sufficiently higher than that of the material constituting the substrate surface and having a size sufficiently small for growing a single nucleus, then forming a substantially monocrystalline semiconductor layer of a first conductivity type on said substrate surface, including a step of forming a nucleus by deposition on said different material and growing a crystal from said nucleus, and growing, on said monocrystalline layer, a substantially monocrystalline semiconductor layer of a second conductivity type. Such polycrystalline solar cell has been shown to have a small thickness, a sufficiently large crystal grain size, and a high energy conversion efficiency.
Such solar cell and the method for producing the same are disclosed, for example, in the European Patent Office Laid-Open Application No. 0276961.
In a polycrystalline semiconductor, however, a number of crystal grain boundaries (hereinafter simply written as grain boundaries) are formed by many monocrystalline grains of different crystal orientations, and defect energy levels are formed in the forbidden band because of the presence of atoms with dangling bonds at such boundaries. The characteristics of the semiconductor device are closely related to the defect density of the prepared semiconductor layer, and the grain boundaries not only have the above-mentioned defect levels but also tend to induce segregation of impurities, both of which give rise to deterioration of the characteristics of the device. Therefore, with a polycrystalline semiconductor, the characteristics of the obtained device are generally considered to be significantly affected by control of the grain boundaries. More specifically, in order to improve the characteristics of a semiconductor device utilizing a polycrystalline semiconductor layer, it is effective to reduce the number of grain boundaries present in the semiconductor layer. In the method for forming the device shown in FIG. 3, the number of grain boundaries can be reduced by increasing the grain size. In this method, the first substantially monocrystalline layer is immediately covered by the second substantially monocrystalline layer, so that the solar cell is composed, as shown in FIG. 3, of a substrate 301, a silicide layer 302, an insulating layer 303, continuously formed polycrystalline silicon films 304, 305, a transparent conductive layer 306, all in a stacked structure, and a current collecting electrode 307. In such configuration, though the number of grain boundaries is reduced in comparison with that in an ordinary polycrystalline semiconductor of smaller gain size, certain grain boundaries 308 are still included at the junction, as such boundaries are inevitably formed at the contact of single crystals, each grown from a single nucleus formed on the small-sized different material constituting the nucleation surface. For this reason, the open circuit voltage of such a device based on a polycrystalline semiconductor is lower than that based on a monocrystalline semiconductor. Therefore, the conventional configuration based on polycrystalline silicon tends to show a high level of dark current resulting from recombination and a relatively low level of photocurrent.
Also, the p-n junction is usually formed in the vicinity of a light-receiving surface of the semi-conductor layer. In the case of a polycrystalline semi-conductor, active grain boundaries 203 in an n (or p) area 201 and a p (or n) area 202 are included in the p-n junction as shown in FIG. 2, thus causing recombination. Therefore the dark current increases significantly in comparison with that of a monocrystalline semiconductor, eventually leading to the deterioration of electrical characteristics, particularly a decrease in the open circuit voltage. Also, the presence of many grain boundaries deteriorates the photosensitivity, thus decreasing the photocurrent. For these reasons, the open-circuit voltage of an ordinary solar cell based on polycrystalline silicon has not exceeded 0.5 V unless certain measures, such as hydrogen passivation, are adopted.